A number of impact sensors have heretofore been proposed for activating vehicle restraint systems. In general, such sensors are of a type that provide a digital output--i.e., an output that turns either on or off--when acceleration/deceleration forces applied to the sensor overcome inertia and biasing forces within the sensor. For example, above-noted U.S. Pat. No. 5,177,370 discloses a number of impact sensor configurations that provide a digital on/off output when impact acceleration/deceleration forces applied to the sensor exceed the sensor design threshold. In the various embodiments disclosed in such patent, the sensor on/off output signal is provided by a Wiegand wire sensor, a digital Hall-effect sensor or electromechanical switch contacts. In each embodiment, the sensor output signal is generated by a permanent magnet when movement of the magnet within a non-magnetic cavity overcomes the internal threshold of the Wiegand wire sensor or the Hall sensor, or when the magnet comes into physical contact with the electromechanical switch sensor.
Digital output sensors of the described character are particularly useful in applications in which a number of sensors are distributed around the vehicle for sensing vehicle impact at a number of locations and/or directions. However, the current trend in the automotive industry is toward so-called single-point sensors--i.e., a single sensor disposed in or near the passenger compartment of the vehicle. Such single-point sensor systems have the advantages of economy over multi-sensor systems because of the reduced sensor cost, and of experiencing the same acceleration/deceleration forces as are experienced by the vehicle occupant(s). However, because such single-point sensors are disposed at vehicle positions remote from impact sites, it is necessary that the sensor and sensor output-responsive system be able to predict a need for activating the occupant restraint system before the high impact forces are actually manifested at the passenger compartment, and sufficiently in advance that the restraint system should be activated at the appropriate time to restrain the vehicle occupants. For example, an air-bag supplemental restraint system should be activated sufficiently early that the bag becomes fully inflated as the vehicle occupant meets the bag. Thus, simply stated, it is necessary in such single-point sensor applications that the sensor and sensor output-responsive electronics be able to predict the need for activating the restraint system well in advance of application of full impact forces on the sensor, while at the same time discriminating against lesser vehicle impacts that do not require activation of the restraint system.
It has been proposed to employ conventional accelerometers, such as piezoelectric and micromachined accelerometers, in such single-point sensor vehicle restraint systems. However, manufacture of such conventional accelerometers requires application of sophisticated micromachining techniques, undesirably increasing both the cost and complexity of the sensors. Furthermore, such sensors have high frequency response, which in some cases must be electronically filtered, making the output more difficult to analyze in the necessarily short time duration required. That is, the frequency response of such sensors is much higher than that of the vehicle occupants, which can be a disadvantage in this application. This problem is illustrated, for example, in Gioutsus, "A Predictive Based Algorithm for Actuation of an Airbag," Sensors and Actuators, SAE (1992) and Piskie et al, "Automobile Crash Modeling and the Monte Carlo Method," Sensors and Actuators, SAE (1992).
It is therefore a general object of the present invention to provide a vehicle restraint system, particularly an impact sensor for a single-point vehicle restraint system, that is able to predict a need for system activation in an accurate and timely manner, while being substantially less expensive to manufacture and easier to test and service than sensors heretofore proposed. A more specific object of the present invention is to provide an impact sensor, which finds particular utility in single-point sensor vehicle restraint system applications, in which physical parameters of the sensor and choice of magnet material may be readily varied at manufacture in order to adapt or tune the sensor to the desired system application, and which employs mature technology.